The present invention relates to a vibration sensor. The present invention relates in particular to an insulating disk for a vibration sensor, such as a knock sensor, for isolation of a piezoelectric disk.
Vibration sensors are known in various embodiments. For example, German Patent No. 44 03 660 describes a vibration sensor having a pressure sleeve which is used with knock sensors for internal combustion engines. In the case of the known vibration sensor, a pressure sleeve is attached fixedly via a contact area to the component whose vibration is to be detected. The vibration to be detected here includes knocking sounds of an engine in operation, the sounds being conveyed via the pressure sleeve to a piezoceramic vibration sensor used as a knock sensor and being converted to an analyzable output signal.
German Patent No. 41 23 786 describes a knock sensor for an internal combustion engine. As diagramed schematically in FIG. 7, the knock sensor has a pressure sleeve 2 in a housing (not shown), the outside of the pressure sleeve having a piezoceramic disk 3 and a seismic mass 4. Seismic mass 4 acts on piezoceramic disk 3 via a spring element 5, which may be prestressed by a threaded ring 6. As shown in FIG. 7, piezoceramic disk 3 is situated between two: contact disks 7 delivering a signal to an analyzer unit via an integrated connector plug (not shown). To guarantee adequate isolation, insulating disks 8 are situated on the outsides of contact disks 7.
However, it is also possible for small shavings 9 (metal flakes) to enter the interspace between pressure sleeve 2 and piezoceramic disk 3. These may be flushed to these locations when screwing on threaded ring 6 or when molding the sheathing onto housing 10, for example. This may result in a connection between piezoceramic disk 3 and the pressure sleeve and/or the seismic mass, possibly resulting in a short circuit. Insulating disks according to the related art have the same inside diameter as the piezoceramic disk and the seismic mass. Therefore, the distances between the individual parts and pressure sleeve 2 are constant. To prevent a short circuit, the insulating disks may be made so thick that small shavings do not have any opportunity to establish a connection between: contact disks 7 and seismic mass 4 or pressure sleeve 2. However, this is a disadvantage for acoustic reasons, in particular, because then it is no longer possible to pick up vibrations optimally.
The vibration sensor according to the present invention for direct or indirect mounting on a vibrating component has the advantage over the related art that it easily prevents a short circuit between the contact disks, or a piezoelectric disk and a seismic mass, or a pressure sleeve. This is achieved according to the present invention by the fact that an inside diameter of the insulating disk is smaller than an outside diameter of a cylindrical area of the pressure sleeve. The inside diameter of the insulating disk is selected so that the disk may nevertheless be threaded easily onto the pressure sleeve. After threading the insulating disk onto the pressure sleeve, the inside circumferential edge of the insulating disk projects out of the plane of the insulating disk, so that the inside circumferential edge is situated at least partially between the components causing the contact. This greatly improves the isolation of the vibration sensor and also improves isolation when affected by moisture.
The insulating disk preferably has at least one slit on the inside diameter which has a predetermined length. This yields a simple means of threading the insulating disk onto the pressure sleeve, while also facilitating uprighting the inside circumferential area of the insulating disk out of the plane of the insulating disk.
According to another embodiment of the present invention, the insulating disk has a slit which is designed continuously from its inside diameter to its outside diameter. Therefore, when the insulating disk is threaded onto the pressure sleeve, it is also widened and closes again automatically due to its inherent elasticity when it has reached its final installed position. In this embodiment, a small-remaining slit is insulated only by the injection molding plastic after sheathing the housing, but in comparison with the previous situation in the related art, the critical volume in which a short circuit is possible is much lower.
To guarantee simple manufacturing of the slit on the insulating; disk, the slit is preferably designed in a V shape.
To particularly facilitate threading of the insulating disk onto the pressure sleeve, the slit in the insulating disk runs in the direction of a center line of the insulating disk.
Easy threading of the insulating disk onto the pressure sleeve may also be achieved by the fact that the insulating disk preferably has a ring-shaped circumferential edge, so that the inside circumferential edge of the insulating disk in which one or more slits are formed projects out of the plane of the insulating disk. This also achieves the result that the insulating disk designed in this way is arranged more easily between the piezoelectric disk and the pressure sleeve.
According to another preferred embodiment of the present invention, a conical area is formed on a transitional area of the pressure sleeve between a cylindrical area and a flange-like edge. This conical area facilitates centering of the elements threaded onto the pressure sleeve and holds them in position, even when plastic sheathing is being injected around the housing. In addition, the conical area of the pressure sleeve supports the arrangement of the insulating disk, or more precisely the internal circumferential edge of the insulating disk between the pressure sleeve and the contact elements of the vibration sensor. Thus, the insulating disk according to the present invention adapts itself to the conical area of the pressure sleeve. Providing slits in the internal edge area advantageously also achieves the result that the partial areas of the internal edge area separated by the individual slits overlap at least partially. This makes the isolation even more reliable.
To permit even better adaptation of the insulating disk to the conical area of the pressure sleeve, the insulating disk preferably has two ring-shaped circumferential edges forming a conical area between them. The conical area of the insulating disk especially preferably has the same pitch as the conical area of the pressure sleeve. This makes it possible to ensure tight contact of the insulating disk with the pressure sleeve.
The two insulating disks of the vibration sensor are preferably designed identically. In this way, uniform isolation may be achieved in all areas of the vibration sensor.
The insulating disk according to the present invention especially preferably has an even number of slits, e.g., eight slits distributed symmetrically on the insulating disk.
The insulating disks may preferably also be designed to have a larger outside diameter than the contact disks or the piezoelectric disk. This also improves isolation in the outer area of the piezoelectric disk. It is also conceivable for the outer circumferential area of the insulating disk to be bent in one direction.
A vibration sensor or an insulating disk is thus made available according to the present invention, providing an improved isolation and insulation effect, in particular with regard to a conducting connection due to metal shavings or flakes or because of moisture. This may be achieved through the especially simple measure of a smaller inside diameter of the insulating disk in comparison with an outside diameter of a cylindrical area of a pressure sleeve of the vibration sensor. This also makes it possible to achieve an improved isolation effect in a surprising and simple manner, with little or no additional cost in manufacture and assembly.